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Astro Midterm 5
| Question | Answer |
|---|---|
| Parsec | the distance from Earth an object would have to be in order to have an observed parallactic angle (p) of 1 arc second (using a baseline of 1 AU) |
| The distance to a star in parsecs | is equal to the inverse of the parallactic angle in arc seconds if and only in the parallactic angle (p) was measured with a baseline of 1 AU |
| What is proper motion in astronomy? | Proper motion refers to the physical movement of stars in space. |
| What is radial motion? | Radial motion is the movement of a star along the line of sight, which cannot be seen visually but can be determined via spectroscopy and the Doppler shift. |
| What causes RA and DEC motion in stars? | RA and DEC motion is caused by the motion of a star in the plane of the sky. |
| What types of objects have detectable proper motion? | Only nearby and/or fast-moving objects have detectable proper motion. |
| Space velocity | is the actual velocity in 3D space |
| Tangential Velocity | is the component perpendicular to the line of sight. Cannot be measured directly. What we measure is motion across the sky in arcseconds per year called proper motion |
| If two stars have the same tangential velocity | then the more distant star shows a smaller proper motion |
| A stars proper motion is related to | both its tangential velocity and its distance. Only nearby and or fast moving objects have detectable proper motion |
| Einsteins theory of relativity | nothing can travel faster than the speed of light |
| Photons | have no mass and cannot be brought to rest. Do have energy |
| Amount of diffraction proportional to | wavelength/gap size |
| Visible light | 400nm-700nm. Is the range of light you can see when you pass a beam of white light through a prism. 700 is red, 400 is purple, green is 550, 450 is blue |
| Infrared radiation | 100 micro m-800nm |
| radio waves | greater than 100 micro meters |
| UV | 400-1nm |
| X ray | 1nm-1*10^-4nm |
| Gamma rays | less than 1*10^-4 nm |
| If the object is moving towards you | the wavelength is blue shifted. Corresponds to a negative velocity. Wavelength gets smaller, frequency gets larger, colour of light moves to the blue part of the spectrum |
| If the object is moving away from you | The wavelength is red shifted. Positive velocity. Wavelength gets longer, frequency gets smaller. Colour of the light moves to the red part of the spectrum. Distant objects in universe are this. MEASURED WAVELENGTH IS LONGER THAN THEORETICAL WAVELENGTH |
| What is luminosity? | The total amount of energy emitted in all directions per second (J/s or W). Intrinsic to the source |
| Is luminosity dependent on distance to the observer? | No, luminosity is independent of the distance to the observer. |
| How does a 100W lightbulb's luminosity behave in different locations? | A 100W lightbulb has the same luminosity regardless of location, such as on the moon or in Nose Hill Park. |
| What is flux? | Flux is the amount of energy measured in an instrument with a collecting area of 1m^2 (J/sm^2 or W/m^2) from a source that is emitting its radiation isotropically. |
| What does flux depend on? | Flux is dependent on distance. |
| How is flux related to brightness? | Flux is related to how bright an object appears to an observer that is a certain distance from the source. |
| Intensity | is the amount of energy measured in an instrument with a collecting area of 1m^2 from a source emitting it's radiation from a surface area of d(omega)/sr. Intensity is related to the "surface brightness", independent of the distance to the observer |
| Magnitudes of stars | mag 1 is the brightest, mag 6 us the faintest object detectable with our eye. FAINTER STAR HAS LARGER MAGNITUDE |
| M | absolute magnitude. Its the apparent magnitude an object would have if it were seen from a distance of 10pc. Related to luminosity |
| Magnitudes and wavelengths | apparent and absolute magnitudes can also be defined for different wavelengths, stars will have different flux levels at different wavelength. So their luminosity/flux/magnitude will depend on what wavelength we are observing. common one is V band |
| IR colour filters | J (1.22 micro meters), H (1.63 micro meters) and K (2.19 micro meters) filters are common in the near IR |
| B-V>0 | mean fainter in the blue than in the red. So this object would appear reddish in colour |
| B-V<0 | mean brighter in the blue than in the red. So this object would appear blueish in colour |
| Colour indices | different stars that radiate according to Planck's law have different B-V depending on their temperature |
| Bolometric magnitude | measure star's radiation at all wavelength (corresponding to its total luminosity-L) or can get from Bolometric Correction |
| Bolometric Correction | BC is around 0 for solar type stars (for the sun it is 0.1) , BC always>0 for both hotter and cooler stars since mbol<mv. BC is ALWAYS greater than 0 because total amount of radiation has to be more than 1 wavelength |
| Specific intensity | Basically the same thing as flux but limited to those photons heading in a particular direction, its flux per solid angle. We only really care about the radiation shining into the telescope area. Iv |
| In isotropic radiation | the net flux passing through the surface is 0, there will be as much radiation entering the surface as leaving it |
| Specific Intensity is sometimes called | surface brightness. IT IS INDEPENDENT OF DISTANCE |
| Radiative transfer | Since intensity is independent of distance, changes in the measured intensity must be a result of radiation interacting with matter |
| kv | absorption coefficient, a property of the material. Wavelength dependent. For radiation transfer |
| What is optical depth? | Optical depth (Tau) is a measure of how much material the observer is looking through and how opaque that material is. Tau and S are defined in opposite directions |
| What is the value of Tau at the observer? | Tau v=0 at the observer. |
| What is the value of Tau at the source? | Tau V=max at the source. |
| What does Tau measure in terms of light? | Tau measures how far we see into the sources and is related to the extinction (the amount of light absorbed). |
| Does optical depth relate to optical wavelengths? | No, optical depth has nothing to do with optical wavelengths. |
| Optically thick | tau>1, even if you're not observing at optical wavelengths |
| A | number of magnitudes of extinction. Av- refers to the number of magnitudes of visual extinction |
| Extinction is dependent on what | Extinction is wavelength dependent. Material tends to be less absorbant/extincting to longer wavelengths of light. |
| Atmospheric extinction | Earth's atmosphere can also absorb radiation. Due to dust in atmosphere but also due to absorption by molecules. It depends on composition (k) and depends on path length (ds), also wavelength |
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